Panel calculation Quiz

[tce]

 I've got a 70w panel and 5 hours of sun daily. Each cell phone charger is rated at 100-240VAC 125 ma input / 5.7vdc 800ma output.

Assuming that it takes 4 hours to charge a phone , how many phones can be  charged at one time. If I were to guess, I'd say about 4 to 5 phones, given that each charger consumes some 12 watts or so of power.

Since charging a phone battery is not linear, I am curious to know if there's a formula for designing a solar phone charging station.

Does one use the wattage of the unit based on input va, or does one use the output va for the calculations.

thanks

tc

[Countryboy]

Your 70 watt panel has DC output.

You forgot to mention how efficient your inverter is to convert the DC to AC, which your battery chargers use.

You have left out too much critical information for anyone to be able to give you a correct answer to the scenario you pose.

An intelligent person would not use your battery chargers.  Since the panel has DC output, they would simply use a transformer to drop panel voltage to 5.7 VDC and hook to the batteries.  (5.7 VDC is 5.7 VDC.  It doesn't matter if that voltage is from a battery charger, or if it is from a different DC source.)

5. 7 volts at 800 ma is 4.56 watts.
   
70. watts, divided by 4.56 is 15.35.
    
    

So, you can charge 15.35 batteries in 4 hours, and provide 1/4 charge to 15.35 more batteries in that 5 hours of full panel output.

[wdyasq]

My cell phone charges with 4.5V and less than an amp. If my goal was to charge cell phones, I would build a charger for them. I would not WASTE wattage going up to 120VAC and then transforming down a rectifying to "X" volts for a small cell phone battery.

The cell phone I have also has a 12V charger I use most often. I plug it in and as I drive it charges the cell phone. IF I were using a panel to charge cell phones and didn't care about the minor losses, I would hook the 600ma 13.8 V Panel I have up to the 12V phone adapter. If I had a 6V - or even 4.5V panel, that would probably work also.

But, the choice is yours. you may be as wasteful as you care.

Ron

[nothing to lose]

As stated.

"You have left out too much critical information for anyone to be able to give you a correct answer to the scenario you pose."

For instance how many teenagers are involved in the cell phone useage in that area?

 If more than 3 then no phone will ever reach full charge anyway due to excess usage.

If the charging station is near a mall, it only takes 1 teenager to keep 20 phones from ever charging as they will be talking to friend A in aisle D about shoes on one phone, and friend B in aisle C about socks on another phone, and they are looking at Jeans in aisle E of the same store at the same time.

So when one phone goes dead and another has charged enough to dial out they will be swapping phones, none will ever charge fully durring the day.

I seen it happen, not the charging part, but the 3 teenagers on 4 and 5 cell phones in same store 1-3 aisles away from each other. I felt sorry for the one poor kid, 2 ears but only one phone, poor thing.

And some adults are as bad, I know some!

As far as the kids, they are too dumb to be embarrased. I once run acrossed this and as I was walking around thinking "darn that's ignorant" one kids cell phone went dead and she was frantically trying to dailout. I asked her her name, she said Barb or such, I simply said loud but reasonable, "Barbs phone is dead and she'll call back later", I got 2  replies "OK, we'll talk to her later" and dumb Barb said "Gee thanks"... Duhhhh!!!

And that is TRUE!

[tce]

Thanks for the replies.  I am aware that an AC inverter is wasteful. a Modified sine wave inverter typically gets no more than 85 % efficient. The 70w, therefore, quickly becomes a 60w panel ( 70 x.85).

What confuses me is the specs on a typical AC phone charger. If it states 100/240 AC 125 ma, I can only assume that this is the REAL consumption--not the output voltage and current.

The difference is, of course, heat loss.

A dc charger, too, is about 90% efficient. But there's no question that going from AC to DC and then using Ac phone chargers is inefficient.

So, to go to my original question, I guess a charger that has an output of 5.7vdc 850ma

(4.8w) while having an input of 100/240 vac 125ma still has to be considered as consuming 12.5w  (100 x .125)---not 4.8 watts.

The answer to my question is then:

59.5 w / 12.5 w = 4.76 phones can be charged  assuming that they require 4 hours of charging time and the panel gets an equal amount of daily sun.

What I still don't know is the total consumption of a charger, given that the charging rate is not linear, but drops as the phone battery fills up.

tc

[wdyasq]

As long as we are speaking efficiency, let's talk about that "70 Watts". Panels are rated at higher than battery voltage "70 Watts Panel" at ~20V equals 3.5 Amp going into  a 12V panel - net is 12V X 3.5A = 42W effective or ~58% efficient.

There will be some loss from the battery charging. THEN you can start accumulating the losses of inverter, charger and such. Of course I'm sure you have not forgotten to factor in the 'cosine factor'. Well, in reality I feel sure that has been forgotten too. Anyway, all of those can be multiplied to get best possible efficiency.

Ron

[ghurd]

The inverter efficiency is probably rated at 90% of the rated load.  It will not be nearly that high with a small load.  Charging all the phones at one time would help.

A good quality inverter will help.

A 70W PV will charge a LOT of cell phones.

My old phones were more efficient than my new phones.  My new phones get a 1 hour charge before it says 'charging complete', about every 3 days.  The new charger says "100-240, 0.2A, Output 5.0VDC, 1A".  I personally would consider 6 cell phones like mine an insugnificant drain for a 70W PV feeding an 85AH battery.

My old phone (1 at a time) would charge from a 50W inverter and a 7AH battery.  The battery would actually gain some charge when a VW PV was in full sun, even with the inverter powering the cell phone charger.  Meaning the average battery draw was less than the 275ma the panel puts out.  (some small inverters draw more than that with no load, so the choice of inverter is important)

Battery draw, new phone ('Audiovox CDM-8910'), same inverter, 90ma no load draw, 100ma with just the charger, 650ma at the start of charging the phone, 600ma 3 minutes later,  510ma at 6 minutes in, 420ma at 10 minutes, 240ma at 30 minutes, 190ma at 45 minutes,  "Charge Complete" at 54 minutes and 107ma leaving the battery.

G-

[ghurd]

A bit more.   5 hours a day (your figure), 70W output of about 4.2A, 21AH / day, minus 10%, minus 1AH / day for the battery self discharge, leaves about 18AH / day.

And ~300maH for a cell phone means about 60 cell phones per day. Every 3rd day for a charge means about 180 cell phones like mine.

The numbers get even better if a few phones were charged at a time, because the 100ma no load draw is counted for every phone and it would not be such a large percentage if 5 phones at a time were charged from the same inverter.  The math will not stand up to intense scrutiny, but the concept is valid.

G-

[tce]

what confuses me is the input rating of the charger. If, as the charger placard indicates, the charger is rated at 100-240AC  125 ma , then I have to assume that when this unit is charging a phone it's using up as much as 12.5 watts. What scares me is the difference between input rating and output capacity.  I am assuming that the current rating of 125ma refers to the 100ac volts, so that  at 240 the current draw is considerably less. Nevertheless, it's the input wattage--not the output-- that I am concerned about.  This is where those folks who advocate going from dc to dc are correct.

I need better clarification as to how one reads these ac chargers and their impact on a solar system.

Thanks very much for the informative observations everyone has made so far....

tc

[ghurd]

Don't get too hung up on the maximum output and input capabilities of the charger.

My wife's car speedometer goes to 160MPH, but we don't know how fast it really goes.

The input rating on the 'charger' has very little to do with the phone uses.

My inefficient example, DC to AC to DC, actually only took 8W peak from the battery, then lowered to 2.4W, even if the max rating of the charger is rated for up to 48W.

I'm not worried about the 1.3W waste on a 70W PV.  Or I would have got the 75W.  

(I am cheap)

If you lay out the money for a 4.8V DC/DC converter, do you do it AGAIN when the contract is up and your new phones need a 6.2V DC/DC converter?  Then that phone breaks and they replace it with a new phone needing 5.0V...

I know a 40W panel that keeps 4~5 cell phones going from a less than good inverter.  And cordless tools, and LED lights, etc.

G-

[cyplesma]

Find out how the specs on a car charger is for cell phones. Start with that.

I just recently used my car charger (12v in) don't know the out to charge my cell phone from a vw trickle charger panel. worked nicely in full sunlight for an hour. usually takes 1.5 hours for the same car charger cord to the phone.

don't think this helps with calc, but like I said start with the specs of car charger cord.